Leaf spot

small round to angular spots on the oldest, lowest leaves — the ones nearest the ground. They start pin-head size, are brown, tan or black, and many have a yellow ring (halo) around them like a target.

the spots grow, some show rings within rings (like a tiny archery target), then they join up into big dead brown patches. The leaf yellows all over, dries out and falls off. The disease climbs from the bottom leaves up toward the top, so a plant can be bare at the bottom and green at the top.

after warm, humid weather with frequent rain or heavy dew, and in crowded, dense stands where the leaves stay wet and air can't move. Old infected leaves lying on the soil are the seed of next season's attack.

with hail or insect chewing (torn edges and holes, not neat round spots with a halo), with plain yellowing from hunger (whole leaf pales evenly, no distinct spots), or with sun-scorch (dry patches at leaf tips and edges, no rings and no yellow halo). True leaf spot makes separate, rounded spots with a coloured edge that later merge.

start looking as soon as the crop is growing fast and leafy (the development stage) and keep it up right through the middle and late season until harvest — these are the stages the disease hits hardest. Check within a day or two after every rain or a heavy dewy morning, because that is when new spots appear.

cross the field in a big "W" shape so you cover all four corners and the middle, not just the edge by the path. Make about 10 stops along the W.

at each stop bend down and look at the lower and middle leaves of a few plants — the bottom of the plant, where the disease always starts. Turn leaves over and look closely for the first tiny spots with a halo.

if you find spots on about 1 in 5 of the lower leaves (roughly 2 leaves in every 10 you check), or you see the spots climbing off the bottom leaves onto the middle of the plant, or lower leaves starting to drop before the crop is ripe — that is your threshold. Act now, while it is still low on the plant; do not wait for the top leaves to catch it.

Rotate: don't plant the same crop on the same land two seasons running — give it a 2-year break so the fungus in the old trash dies out. Clean up: after harvest, gather and burn or bury the infected leaves and crop trash so there are no spores to splash up next season, and always plant clean, healthy seed.

Where your local seed supplier or extension officer offers a variety bred to tolerate leaf spot, choose it — it is the cheapest lasting protection and needs no spraying. (Resistant means the plant fights the fungus off on its own.)

Space plants wider and weed well so air moves through and the leaves don't stay wet — the fungus needs wet leaves to grow. Water at the base, not overhead, water early in the day so leaves dry by night, mulch the soil to stop rain splashing spores up, and never walk through or work the crop while the leaves are wet — you carry spores from plant to plant on your hands and clothes.

A protectant fungicide (a spray that coats the leaf and stops new spores taking hold — it does not cure spots already there, so it must go on early, at the very first spots, and be repeated through wet weather). Safety, every time you spray: cover your skin, eyes, nose and mouth; spray in the cool of morning or evening with the wind at your back so it blows away from you; keep it off food, drinking water, children, animals and bees; and after each spray wash yourself and your clothes. Respect the pre-harvest interval — the number of days the label says you must wait between the last spray and picking or eating the crop. To stop the fungus getting used to one spray (resistance), rotate between different fungicide types rather than using the same one over and over.

tighten up — spray protectant on a shorter cycle in the wet spell, strip and destroy the worst-hit lower leaves, and make sure the canopy is open and dry. But if the plant is already mostly bare and the spots have reached the top before the crop is mature, spraying will not bring the leaves back — cut your losses: harvest whatever you can, then clear and destroy every scrap of infected trash, and next season rotate to a different crop and start with a resistant variety and clean seed so you don't carry the disease forward.